The present invention concerns a silver ink for use in forming the electrodes of a low temperature fired multilayer ceramic capacitor.
Multilayer ceramic capacitor (MLC) devices are well-known in the electronics industry. MLC devices are commonly fabricated from dielectric green tapes and electrically conductive inks that usually contain mixtures of the metals silver and palladium. Typically, the electrically conductive silver-palladium inks are screen printed in a desired pattern upon the green tapes. The printed green tapes are stacked together to form a registry and then laminated under pressure to form a monolithic structure. The monolithic structure is then fired (conventionally at a temperature of about 1250xc2x0 C.) to form the MLC device.
In recent years, ceramic dielectric materials have been developed that can be fired in the range of about 880xc2x0 C. to about 925xc2x0 C. Because of the lower firing temperatures, attempts have been made to lower the palladium content of the conductive inks used to form the electrodes. Palladium is much more expensive than silver, so minimizing its use can result in large cost savings. Unfortunately, attempts to formulate conductive inks comprising 100% silver have been met with limited success. The primary problems. have been poor adhesion, silver migration, silver diffusion, and/or decreased electrical performance.
The present invention provides a new and improved conductive silver ink for use in forming the electrodes of a low temperature fired MLC device. The ink of the present invention displays excellent adhesion and electrical properties. In a preferred embodiment, the ink comprises silver powder, a vehicle and an inhibitor. The silver powder comprises particles having a maximum particle size of less than or equal to about 1 xcexcm. The inhibitor preferably comprises a barium titanate based ceramic composition comprising particles having a maximum size of less than or equal to about 0.9 xcexcm. The present invention also provides a novel low temperature fired multilayer ceramic capacitor and method for forming the same.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
The conductive ink of the present invention comprises silver powder, a vehicle, and an inhibitor. The ink of the present invention is free of palladium, gold or any metal other than silver. Preferably, the ink also includes a stabilizer and/or antioxidant. Other additives such as dispersants and thixotropes may also be included.
The silver powder used in the ink according to the present invention may be obtained from a variety of commercially available sources. For example, a suitable silver powder is available from the Engineered Powders Division of Technic Inc. of Woonsocket, Rhode Island under the trade designation SILSPHERE(trademark) 514 silver powder. Such powder displays an average particle diameter of from about 0.4 to 1.0 xcexcm and a purity in excess of about 99.5% by weight. Silver powder may be obtained from other commercial sources, however the utilization of a silver powder having an average particle size of less than or equal to about 1 xcexcm has been found to be a critical aspect of the ink.
The inhibitor employed in the present invention preferably comprises a ceramic metal oxide. Preferably, such inhibitor comprises a barium titanate based material. More particularly, preferably the inhibitor has a composition the same or similar to the dielectric powder utilized to form the green tape.
The inhibitor serves to help promote compatible expansion rates and adhesion between the dielectric and electrode layers subsequent to firing. The inhibitor preferably has a maximum particle size of less than or equal to about 0.9 xcexcm. As previously noted, a suitable source for the inhibitor may be the dielectric powder used to form dielectric tape. It will be appreciated that milling of the powder may be required so as to provide the proper particle size for the inhibitor.
One example of a suitable dielectric powder that may be used to form green tape as well as be used as the inhibitor in an ink composition according to the present invention is a low temperature dielectric powder comprising primarily a mixture of barium titanate and neodymium oxide sold by the Ferro Corporation of Cleveland, Ohio, under the trade designation ULF X7R. Such powder fires in the 880xc2x0 C. to 925xc2x0 C. temperature range. Such powder has a particle size (d50) of about 0.84 xcexcm. The inhibitor may also be supplied in the form of a coating on the silver powder. ZrO2 coated silver powders are commercially available from the Degussa Corporation of South Plainfield, N.J.
The vehicle used in the ink composition according to the present invention preferably comprises a mixture of resin and solvent. The preferred resin/solvent mixture comprises a 10 weight percent solution of ethyl cellulose in an 80/20 (wt. %) mixture of toluene/ethanol. Other suitable solvents for use in formulating the vehicle include, for example, ethylhexanol and/or an dearomatized aliphatic sold under the trademark Exxsol D70 by Exxon Chemical Company.
The ink preferably comprises a stabilizer/antioxidant such as, for example, Ciba-Geigy""s Irganox MD-1024. Of course, any one or combination of conventional stabilizers/antioxidants may be employed. Stabilizers/antioxidants serve to preserve the shelf life of the ink helping to prevent any time related deterioration of the system. The ink may also include other conventional additives such as thixotropes and dispersants. These types of additives are commonly employed in electronic inks and are well-known in the art.
The ink of the present invention is prepared by mixing the various components together, for example, using a three roll mill. After milling, the ink composition is filtered to remove large particles and agglomerations. Preferably, the filter has a maximum opening size of about 1 xcexcm. Filtering is critical in order to ensure that the ink does not contain silver particles in excess of 1 xcexcm.
Low temperature fired MLC devices may be formed using the ink composition of the present invention. Such capacitors are formed by providing multiple layers of green dielectric tape and then depositing the ink upon the layers of tape. The ink may be deposited using any one of a variety of conventional techniques, such as screen printing. Once the tapes are coated with ink, the tapes are stacked in a registry and laminated to form a monolith. The monolith is then fired at a temperature of from about 86 to about 940xc2x0 so as to form a capacitor. Applicants hereby incorporate by reference U.S. Pat. No. 5,296,426 (especially col. 7, lines 26 to col. 8, line 3) and U.S. Pat. No. 4,379,319 (especially FIG. 2) for their general teachings regarding the fabrication of MLC devices.